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HYDROLOGIC RESIDENCE TIME CONTROLS THE FATE OF CARBON AND NITROGEN AT A LAKE-GROUNDWATER INTERFACE

Groundwater-surface-water (GW-SW) interfaces are important regulators of biogeochemical processes. In lakes with highly permeable sediments, GW-SW exchange can control lake-level influx and removal of compounds such as nitrate, a common pollutant in lakes and groundwater. To test how hydrologic conditions at the GW-SW interface influence nitrate uptake, we conducted a series of experiments varying hydrologic residence time in down-welling lakebed sediments of a lake on Cape Cod, MA. We added isotopically-labelled 15N-nitrate to distinguish uptake from denitrification, quantifying the 15N of nitrate, nitrite, dinitrogen gas, and nitrous oxide. We additionally added acetate, a labile carbon compound, to test for carbon limitation. We found that acetate addition triggered the GW-SW interface at the lake bed to switch from net production to net removal of nitrate. Together with increased residence time from experimental decreases of down-welling, this resulted in a 20-fold increase in denitrification. We conclude that lake hydrologic conditions at the GW-SW interface control nitrate removal. Because lakebed water exchange is driven by seasonal variations in lake and groundwater stage, this provides a robust tool for temporal scaling of nitrate dynamics.

Tyler Hampton (Primary Presenter/Author), Department of Earth and Environmental Sciences, Michigan State University, USA, thampton@msu.edu;


Jay Zarnetske ( Co-Presenter/Co-Author), Department of Earth and Environmental Sciences, Michigan State University, USA, jpz@msu.edu;


Martin Briggs ( Co-Presenter/Co-Author), U. S. Geological Survey, Office of Groundwater, Branch of Geophysics, Storrs, Connecticut, USA, mbriggs@usgs.gov;


Kamini Singha ( Co-Presenter/Co-Author), Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO, USA, ksingha@mines.edu;


Jud Harvey ( Co-Presenter/Co-Author), U. S. Geological Survey, National Research Program, Reston, VA, USA, jwharvey@usgs.gov;


Fred Day-Lewis ( Co-Presenter/Co-Author), U. S. Geological Survey, Office of Groundwater, Branch of Geophysics, Storrs, Connecticut, USA, daylewis@usgs.gov;


John Lane ( Co-Presenter/Co-Author), U. S. Geological Survey, Office of Groundwater, Branch of Geophysics, Storrs, Connecticut, USA, jwlane@usgs.gov;